In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise at least one imageable layer applied over the hydrophilic surface of a substrate. The imageable layer(s) include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged (exposed) regions are removed, the element is considered as positive-working. Conversely, if the non-imaged (non-exposed) regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or a fountain solution and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers.
Negative-working imageable elements useful for making lithographic printing plates are described in numerous publications including U.S. Patent Application Publications 2008/70152 (J. Yu et al.) and 2007/269739 (Nguyen et al.).
Some of these imageable elements are designed to have an oxygen barrier or topcoat layer disposed over the imageable layer. WO 2005/029190 (Gries et al.) describes the use of topcoat layers composed of poly(vinyl alcohol) having a high degree of saponification.
EP Publication 1,708,025 (Arimura et al.) describes negative-working lithographic printing plate precursors N-nitroso-N-phenyl hydroxylamine.